The invention relates to the drive train of a motor vehicle, with an intermediate shaft arranged between a power divider and a side shaft. The invention relates, furthermore, to a method of making a bearing unit for the mounting of a drive shaft of a motor vehicle.
U.S. Pat. No. 4,421,187 describes a drive train of a motor vehicle that includes an intermediate shaft which is interposed between an output bevel gear of a power divider or differential and an axle shaft or side shaft pivotable by means of a compression of the vehicle wheels. In the end region facing the output bevel gear, the intermediate shaft is mounted in the power divider by means of a first bearing. In the end region located opposite the power divider, the intermediate shaft is connected to the side shaft via a constant-velocity joint. The intermediate shaft is further mounted between the first bearing and the constant-velocity joint by means of a bearing unit which, for example, is supported likewise within the power divider or with respect to an engine block.
The mountings of the intermediate shaft are under high mechanical stresses as a result of high effective moments. Due to the connection to the power divider and to the side shaft via the constant-velocity joint, the drive shaft is exposed, in particular, to vibration excitations, for example as a result of a play of the constant-velocity joint. Equally, vibrations of the engine block or transmission block are transmitted to the intermediate shaft via the mountings. To avoid disturbances or impairments in the functioning of components of the drive train and of further components connected thereto, and to achieve a high degree of comfort, the intermediate shaft must be guided radially and axially with the necessary precision. At the same time, undesirable bearing forces should not be introduced into the body by the mountings, since such bearing forces are detrimental to driving comfort or cause sound radiation.
DE 39 11 914 C2 discloses a rolling-bearing arrangement for general uses, in which a flexurally elastic bearing outer ring is guided in one direction in a groove for radial displacements and, in a direction perpendicular to the abovementioned direction, has a play with respect to a bearing housing. DE 197 55 307 discloses a rolling-bearing arrangement for a drive shaft of a motor vehicle, the rolling-bearing arrangement having different rigidities for different load directions in the bearing plane.
DE 39 33 515 C2 discloses an elastic intermediate mounting for a cardan shaft with a fluid filling for flexible mounting. With this intermediate mounting, vibrations with high amplitude and low frequency and with low amplitude and high frequency are damped.
DE 198 09 528 A1 discloses a vibration-reduced shaft bearing for a drive shaft of a differential, by means of which shaft bearing noises caused by bearing play are reduced.
Furthermore, DE 35 11 480 C1 discloses an intermediate bearing for an articulated shaft of motor vehicles. In the intermediate bearing, an elastic intermediate layer, which receives a bearing outer ring, is positioned, free of play, as a result of prestress by means of projections.
DE 39 08 965 A1 discloses a cardan-shaft bearing with an outer and an inner ring which together delimit a gap extending in the axial direction and are supported radially elastically one on the other by a rubber intermediate ring.
An object of the present invention is to propose a drive train with improved mechanical transmission behavior. Another object of the invention is to propose an improved method of making a bearing unit for the mounting of a drive shaft of a motor vehicle.
The objects of the invention are achieved by the invention described and claimed hereinafter. The bearing unit of the present invention has a play oriented radially to the intermediate shaft. As a result, the bearing unit exerts radial forces on the intermediate shaft only after the play has been overcome. At the same time, this means that forces can be transferred in the direction of the body in the radial direction and, conversely, in the direction from the engine to the side shaft only when the play is overcome. As a result, an improvement in comfort and a reduction in sound radiation can be achieved.
The play extends over parts or large parts of the circumference. The invention follows a different path from the customary principle of providing a rotating shaft acted upon by high moments with a fixed mounting and a loose mounting. Instead, for small deflections, the shaft is mounted solely by the bearing arranged adjacently to the output bevel gear of the power divider and is coupled to the side shaft via the constant-velocity joint. Contrary to the usual principle, according to the invention, the basis, in the radial direction, is a self-centering of the intermediate shaft as a result of the one-sided mounting of the first bearing, as a result of the coupling to the side shaft and as a result of the stabilization of the rotational movement of the intermediate shaft by torsion stabilization. The second bearing unit equipped with the play thus serves merely as a “take-up solution” for deflections of the intermediate shaft which exceed the minimum amount.
As a result of the radial play, when radial accelerations occur there is a (highly) non-linear (springing and/or damping) behavior with low or insignificant rigidity or damping during the overcoming of the radial play and with an abrupt increase in rigidity or damping for a closed-up radial play. As a result of the non-linear rigidity, with periodic excitation being assumed, non-linear effects may arise, for example subharmonic and/or superharmonic vibrations or chaotic vibrations. The result of this is that the vibration excitation not only results in a vibration response with one excitation frequency with high amplitude, but the energy is also apportioned to a plurality of frequencies with a lower amplitude.
In particular, as a result of the play, the vibrations of the drive shaft which occur are transmitted, filtered according to frequency components, to the bearing unit. On the assumption of vibrations with identical energy, the amplitude for low frequencies is higher than the amplitude for high frequencies. For example, the extent of the play is dimensioned such that, for vibrations with high frequencies, the low amplitude lies within the play, so that high-frequency vibrations remain unaffected by the bearing unit and the vibrations are not transmitted to the body of the vehicle via the bearing unit. In particular, the mounting is adapted to the vibrations having low frequencies.
Preferably, the bearing unit has a bearing, a bearing carrier and an elastic intermediate layer arranged between the bearing carrier and the bearing. The radial play is provided in the force flux between the bearing and the bearing carrier. According to a preferred embodiment of the invention, the intermediate layer has projections which point radially outwards. The projections ensure a form-locking securing of the position of the intermediate layer with respect to the bearing carrier in the axial direction and/or in the circumferential direction. Accordingly, despite the fact that rigidity in the radial direction disappears as a result of the play, for low displacements the drive shaft can be guided in the axial direction and/or circumferential direction by the projections (even when there is freedom from play). The selection of rigidity for the projections and consequently for the abovementioned directions may be made independently of the rigidity of the bearing unit in the radial direction, with the result that the possibilities for influencing the dynamics of the bearing unit are increased.
According to an aspect of the invention, in a first method step for the making of a bearing unit with an elastic intermediate layer introduced into a bearing carrier, the intermediate layer is vulcanized into or onto the bearing carrier. In such an introduction into the bearing carrier, a connection to an (inner) bearing ring can be made simultaneously with the vulcanization. After the first method step, the intermediate layer is detached from the bearing carrier in a second method step, with the result that the play in the form of a circumferential gap is formed between the intermediate layer and the bearing carrier. Detachment takes place by a cooling of at least the intermediate layer. Due to the cooling, a thermal shrinkage of the intermediate layer is obtained. In this case, it is advantageous if the coefficient of thermal expansion of the intermediate layer is higher than the coefficient of thermal expansion of the bearing carrier. According to the invention, an exactly defined gap can be produced in a particularly simple way, without, for example, (highly accurate) manufacturing steps, remachining steps or (narrow) tolerances being necessary. The play size or gap height may in this case be predetermined by the selection of the temperatures or temperature changes and of the coefficients of thermal expansion of the intermediate layer and of the bearing carrier.
Preferably, the first method step is preceded by a pretreatment of the bearing carrier in order to ensure a subsequent detachment of the intermediate layer from the bearing carrier. This pretreatment may involve, for example, the application of a releasable layer or of a grease layer to the bearing carrier.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.